Item of footwear

ABSTRACT

The present invention relates to an item of footwear having a sole, comprising two or more materials of different compressive resistances, flexibility or densities arranged as a frontal/toe region, a mid foot region and a heel region. The item of footwear comprises: a securing means for securing the item of footwear to a foot of a wearer; and a sole having a midsole which has a frontal/toe region, a mid-foot region and a heel region, the mid-foot region of the midsole being formed from a material different to the material used for the heel region such that the mid-foot region has a lower compressive resistance and/or a lower density and/or a higher flexibility than the heel region, wherein the mid-sole is thin and tapered.

The present invention relates to an item of footwear having a sole,comprising two or more materials of different compressive resistances,flexibility or densities arranged as a frontal/toe region, a mid footregion and a heel region.

Most shoes are purchased because of their aesthetics or their comfort.Comfort shoes are designed to provide comfortable walking and standingfor a user and often include reasonable internal padding, arch supportand comfort soles. In normal use, prior art shoes provide a good stablebase for a foot of a user and this, consequently, reduces the amount ofmuscle force required for a user to stand still or walk, whilstmaintaining good balance.

The inventor has previously found that, by providing a slight, butsafely-measured, instability in the footwear, the amount of difficulty auser experiences balancing whilst walking or standing is increased and,therefore, more muscle activity is required to achieve equivalent goodbalance to when using prior art shoes. In particular, muscles such astibialis anterius, tibialis posterior, rectus femoris muscles of thequadriceps and gluteus maximus have been found to work harder whilstwearing footwear having a slight instability. Accordingly, WO2008/132478 discloses an item of footwear that aids muscle toning andstrengthening whilst a user is standing or walking around as they woulddo normally.

It has previously been believed that the sole of the above-describeditem of footwear would only achieve an instability sufficient to providea muscle training effect if the sole was of a significant thickness(significantly greater than regular footwear). The applicant has to datebelieved that a mid-sole with a thickness at the heel of at least 40 mmis needed to provide the instability effect. However thicker soles areheavier, more costly, and are often considered less attractive.

The present invention provides an item of footwear as claimed in claim1.

The inventor has discovered that a mid-sole comprising heel, frontal/toeand mid-foot regions of differing physical characteristics and having aheel thickness of between 18 to 28 mm, preferably from 23 mm to 25 mmand most preferably approximately 24 mm, can provide an advantageousinstability effect.

A preferred embodiment of the present invention will now be described,by way of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is a top plan view of a sole of an item of footwear according tothe present invention, the figure having additional lines to illustratefeatures of the footwear below the top surface of the sole;

FIG. 2 shows a cross-section through the item of footwear of FIG. 1taken along the line A1-A2 of FIG. 5;

FIGS. 3 and 4 are side elevation views of the item of footwear of FIG.1, respectively showing the lateral view and medial views of the item offootwear;

FIG. 5 is a bottom plan view showing an outsole of the item of footwear;and

FIGS. 6, 7 and 8 are cross-section views through the item of footwear ofFIGS. 1 to 5, respectively taken along the lines of B1-B2, C1-C2 andD1-D2 shown in FIG. 5.

The present invention relates to an item of footwear, such as a sandal,a flip-flop or a closed shoe. In the figures the sole of the item offootwear is shown, but it should be understood that the sole willtypically be provided with a strap arrangement (not shown)or an upper tosecure a foot to the sole.

As shown in FIG. 4, the preferred embodiment of the invention has a sole2 formed from a topsock 2A, a midsole 2B and an outsole 2C. The topsock2A is bonded to the midsole 2B and the midsole 2B is bonded to theoutsole 2C to provide a unitary sole 2. The topsock 2A is provided toreceive a foot of a wearer and is textured to provide a gripping surfacefor the foot of the wearer. The topsock 2A could be a coating applied toan upper surface of the midsole 2B, e.g. a textile coating, or a layerof fabric bonded to the upper surface of the midsole 2B; a moresubstantial insole could be used in place of the topsock. The topsock 2A(or insole) could be integral with the midsole 2B. The outsole 2Cprovides a plantar surface, i.e. a surface which contacts the ground,and may be provided with one or more ridges or grooves (not shown) forproviding grip. The outsole 2C is preferably moulded as a single pieceprior to bonding with the midsole 2B. Since it contacts the ground, theoutsole 2C is formed by a material having good wear characteristics. Theoutsole 2C preferably has a constant thickness of about 4.5 mm.

The midsole 2B is preferably formed with three distinct regions, as canbe seen in the cross-section of FIG. 2. A frontal/toe region isindicated by reference numeral 4. A mid-foot region is indicated byreference numeral 5. A heel region is indicated by reference numeral 6.The three regions of the midsole 2B are formed from two or morematerials of different compressive resistances and/or flexibilitiesand/or densities, with the mid-sole region 5 being provided from amaterial of least compressive resistance and/or density and/or greatestflexibility. Thus the sole 2 is arranged such that application of awearer's weight, during walking, causes instability in the sole whichrequires balance correction by a user. The amount of instability causedis slight, but enough for small balance corrections to be made by themuscles of the wearer's leg.

The mid-foot region 5 of the midsole comprises a first material of afirst compressive resistance and/or density and/or flexibility; thefrontal/toe region 4 comprises a second material of a compressiveresistance higher than that of the first material and/or a densityhigher than that of the first material and/or a flexibility lower thanthat of the first material. Further, the heel region 6 is of a thirdmaterial—different from the first and second materials—of a compressiveresistance higher than that of the first material and/or a densityhigher than that of the first material and/or a flexibility lower thanthat of the first material. It is preferred that the sole 2 has a heelregion 6 and a frontal/toe region both harder than the mid-foot region5. In also preferred that the frontal/toe region 4 is slightly softerthan the heel region 6, but with both the heel region 6 and thefrontal/toe region 4 harder than the mid-foot region 5. By way ofexample, the hardness of the regions of the midsole 2B can measuredusing the Shore scale, the heel region has a Shore hardness of 48, themid-foot region a Shore hardness of 28 and the frontal/toe region aShore hardness of 31.

In preferred embodiments of an item of footwear 1 according to thepresent invention, the frontal/toe region 4 extends over the forwardmost15% to 24% of the length of the sole 2, preferably around 18%, themid-foot region 5 extends over the middle 37% to 53% of the length ofthe sole 2, preferably over the middle 47% to 53% of the length of thesole 2 and most preferably around 50%, and the heel region 6 extendsover the rearmost 29% to 39% of the length of the sole 2, preferablyaround 32%. The lengths of these regions are indicated respectively asA, B and C in FIG. 2.

Whilst it is preferable that the material of the mid-foot region 5 has alower compressive resistance and/or a lower density and/or a higherflexibility than the frontal/toe region 4, when the frontal/toe region 4is thin (as it is in tapered soles), these relative properties are lessimportant. In tapered soles, however, it is still essential that theheel region 6 has a higher compressive resistance and/or a higherdensity and/or a lower flexibility than the mid-foot region 5.

The mid-sole 2B is preferably manufactured by a multi-stage mouldingprocess. Initially the mid-foot region 5 is moulded as an independentcomponent with inclined front and rear surfaces, i.e. in a trapeziumshape when viewed in side elevation. Then the mid-foot region is loadedin a mould with granules of material to form the frontal/toe region andthe heel region and heat is applied to melt the granules and form thefrontal/toe and heel regions in the mould, melded to the preformedmid-foot region. Alternatively liquid material could be injected intothe mould to form the frontal/toe and heel regions. On removal from themould the initially roughly formed midsole expands in volume. It isabraded on its upper and lateral surfaces and a topsock is applied tothe upper surface. The assembly of topsock and midsole is placed in asecond mould in which it is heated and maintained at an elevatedtemperature for a chosen period of time, then it is cooled in the mouldby passing a cooling fluid, e.g. water, around the mould for a chosentime before release from the mould. The released combination of topsockand midsole has the required end shape and the desired surface features,and is ready for the outsole to be adhered to the lower surface of themidsole.

The thickness 19 of the heel region 6 is typically measured as a minimumthickness, i.e. the thickness of the heel region at the lowest point ofthe heel region, this being typically found in the central part of theheel region when viewed in plan view. The thickness of the heel region 6of a mid-sole according to the present invention is in the range 18 mmto 28 mm, preferably from 23 mm to 25 mm, and most preferably around 24mm. It has been found that within this range it is possible to obtain asuitable training effect if the layer is tapered as described below.Above this range, the thickness of the layer will be great enough thatsuch tapering is not required to achieve the effect. Below this range,the thickness of the layer will be too small to achieve a trainingeffect.

As can be seen from FIGS. 2 to 4, the sole tapers along its length fromthe heel region 6 to the frontal/toe region 4. This taper is not alinear reduction in depth along the length of the sole, but is anoverall thinning of the sole from heel to the toe. In fact, the thinningwill not be continuous and some portions of the sole will be of constantthickness. What is important is that the wearer's foot is angled by theshoe such that the wearer's toes are lower than the wearer's heel.

The sole 2 has a lateral side as shown in FIG. 3 and a medial side asshown in FIG. 4. When worn by a wearer, the big toe of the wearer's footis on the medial side of the sole 2.

The taper of the multi-density material layer is typically described byreference to an effective heel height, this being the difference inthickness between the thickness at a centre of the heel region 6 and thethickness of the midsole where it supports the ball of a wearer's foot.The midsole will support the ball of the wearer's foot near thetransition between the frontal/toe region 4 and the arch region 5. Infact, it may be the case that the ball of the wearer's foot extends overthe transition between these regions. For preferred embodiments of thepresent invention the effective heel height is in the range 6 mm to 18mm, more preferably between 6 mm and 10 mm or between 12 mm and 16 mm.Preferred values of the effective heel height are around 8 mm or around15.5 mm.

In the preferred embodiment, the sole comprises a concave recess 15formed in the midsole, seen in FIGS. 1 and 5 and seen by comparing thecross-sections of FIGS. 7 and 10, respectively taken through thefrontal/toe region and the heel region of the sole, with thecross-section of FIG. 8, taken through the recessed mid-foot region. Therecess 15 extends from a part of the heel region 6 just forward of acalcaneus of a foot of a wearer to a point in the frontal/toe regionwhich aligns with the phalangeal metatarsal joint of the wearer's foot,otherwise known as the first ray of the foot. In the illustratedembodiment this means that the recess extends rearwardly along themedial surface of the midsole from a point roughly 40% aft of the frontof the sole to a point roughly 20% forward from the rear of the sole.

In the preferred embodiment of the invention the narrowest width of thelowermost surface of the midsole in the recess 15 will be 61 mm to 67 mmand compared with a width of the uppermost surface of the midsolemeasured at the same point of 67 mm to 84 mm and more preferably from 67mm to 75 mm. Preferably, the recess 15 when measured across thelowermost surface of the outsole at its narrowest point has a width of61.5 mm, which compares with a width measured at the same point on theuppermost surface of the midsole of 71 mm. A further embodiment isconsidered in which the recess 15 when measured across the lowermostsurface of the outsole at its narrowest point has a width of 64 mm,which compares with a width measured at the same point on the uppermostsurface of the midsole of 81 mm.

The width of the lowermost surface of the midsole at its narrowest pointcan be narrower than the width of the uppermost surface of the midsoleat the corresponding point by between 10% and 30%, and preferablybetween 10% and 20% and most preferably by 13%, without adverselyaffecting the wearer of the shoe. The minimum slope of the side of therecess 15 will be in the range of 45° to 80°, and preferably 75°.

The inventor has discovered that a wearer will walk with a gait thatduring a normal stride results in pressure being applied first centrallyin the heel region 6, then along the lateral side of the mid-foot regionof the sole and finally centrally in the frontal/toe region 4. Themid-foot region 5 of the sole does not therefore need to support theuser's foot across the full width of the sole 2, but only along thelateral side of the mid-foot region 5. This means that the weight andthe cost of the sole 2 can be reduced by using less material in themid-foot region 5. Advantageously, the soles of the invention achievethe reduction whilst maintaining a normal plan view outline of theuppermost surface of the midsole, by reducing the width of the midsolefrom it uppermost surface to its lowermost surface such that the widthof the uppermost is greater than the width of the lowermost surface, asdescribed above. Width in this context is defined as the distance acrossthe sole from the lateral side to the medial side in a directionperpendicular to a longest line joining the tip of the frontal/toeregion 4 to the tip of the heel region 6; shown as line 33 in FIG. 5.

The provision of the recess 15 in the medial side surface of the midshoe region enhances the instability effect provided by the sole andthus permits a reduced heel width.

The concave recess in the medial surface of the midsole will typicallybe formed in the last stage of the moulding process formed above, i.e.it is provided as a feature of the second mould. The sole will have ablock form until it is placed in the second mould.

As can be seen from FIG. 5, in addition to the presence of the recess15, the lowermost surface of the midsole 2B is narrower than theoutermost surface of the midsole 2B, around its entire periphery. Insome embodiments the lowermost surface may be narrower than theoutermost surface by from 1.5 mm to 2.5 mm and preferably around 2 mm.

More preferably, however, because the midsole 2B is tapered, this ismore pronounced in the heel region 6 than in the frontal/toe region 4.In such embodiments, the periphery of the outsole is stepped in between2 mm to 6 mm from the uppermost to lowermost surfaces in the heel region6 and between 1 mm and 2.5 mm in the frontal/toe region 4. The outsoleis the same size as the lowermost surface of the midsole.

As can be seen in FIG. 2, the frontal/toe region 4 abuts the mid-soleregion 5 along an interface 7. At the interface 7, the material of thefrontal/toe region 4 is melded to the material of the mid-foot region 5.In a similar way, an interface between the mid-foot region 5 and theheel region 6 is provided at an interface 8. Although the interfaces 7,8could be vertical in nature, it is preferred to have a taperedinterface. In particular, a tapered portion 4A of the frontal/toe region4 overlaps with a tapered portion 5A of the mid-foot region 5 with themid-foot region 5 extending over and above the frontal mid-sole toeregion 4. Further, a tapered portion 5B of the mid-foot region 5overlaps with a tapered portion 6A of the heel region 6, with themid-foot region extending over and above the heel region 6. Preferably,the interface between each region is sloped so as to increase thesurface contact between the regions. In which case, the size of theregions will vary between the upper and lower surfaces of the midsole2B. It is preferable that the frontal/toe region 4 extends over theforwardmost 18% of the top surface and 24% of the lower surface, themid-foot region 5 extends over the middle 52% of the top surface and 37%of the lower surface, and the heel region 6 extends over the rearmost30% of the top surfaces and 39% of the lower surface.

The interfaces are preferably inclined 45° to the horizontal when thesole is on a horizontal surface.

Advantageously, as can be seen in the Figures, the sole 2 isanatomically-shaped. The materials of the frontal/toe region 4, themid-sole region 5 and the heel 6 are arranged to provide a wobbleeffect—slight instability—such that the foot, when stepping across thesole from heel 6 to toe 4 wobbles slightly, altering the balanceposition of a user's foot. The wobble effect can be medio-lateral oranterior-posterior or a combination of anterior-posterior andmedio-lateral instability. In particular, it is the progression ofpressure from the heel 6 through the lateral side of the mid-foot region5, and then to the frontal/toe region 4 that causes the mid-foot regionto flex to produce a wobble and cause slight instability.

The embodiment of sole 2 shown in the Figures is preferably made ofethylene vinyl acetate (EVA). According to the invention, threedifferent grades of EVA are utilised—having different compressiveresistances and/or densities and/or flexibilities—for the differentregions of the midsole to provide the wobble effect. However, it will beunderstood that other materials will provide an equivalent effect andexamples of those materials are other elastomers, silicones, natural orsynthetic rubbers and/or polyurethanes.

It has previously been considered that the instability effect of theshoe requires a broad sole to support the user's foot as he corrects hisbalance. Previously, it was assumed that the maximum width in the uppersurface of the mid-sole in the heel region must be at least 75 mm.

Similarly, it was assumed that the maximum width in the upper surface ofthe mid-sole in the upper surface of the mid-sole in the frontal/toe andarch regions must be at least 98 mm.

However, the inventors have discovered that the user can compensate forthe instability and achieve a suitable training effect even in narrowershoes.

Indeed, it has been discovered that footwear having a maximum width inthe upper surface of the mid-sole in the heel region in the range of 55mm to 68 mm can still achieve the training effect.

Similarly, it has been discovered that footwear having a maximum widthin the upper surface of the mid-sole in the frontal/toe and arch regionsin the range of 80 mm to 92 mm can still achieve the training effect.

The tapered sole described above, is particularly advantageous for usein a closed shoe (e.g. a shoe with an upper cover that encapsulates thetoes and/or foot forward of the arch). This is because is itaesthetically desirable for closed shoes to be narrow (as compared withopen shoes). Narrow shoes constrict the wearer's toes within the cavitydefined between the sole and the upper. To enable such a narrowconstruction, it is necessary to increase the volume of the cavity; bytapering the sole an increase in volume at the forward end of the shoeis achieved.

1. An item of footwear comprising: a securing means for securing theitem of footwear to a foot of a wearer; and a sole having a midsolewhich has a frontal/toe region, a mid-foot region and a heel region, themid-foot region of the midsole being formed from a material different tothe material used for the heel region such that the mid-foot region hasa lower compressive resistance and/or a lower density and/or a higherflexibility than the heel region, wherein: the mid-sole in the heelregion has a thickness in the range of 18 mm to 28 mm; and the mid-solehas a thickness in a part of the region for supporting a ball ofwearer's foot which is between 6 mm and 18 mm less than the thickness ofthe mid-sole in the heel region.
 2. The item of footwear of claim 1,wherein the frontal/toe region is formed from a material having highercompressive resistance and/or higher density and/or lower flexibilitythan the mid-foot region.
 3. The item of footwear of claim 1, whereinthe thickness of the mid-sole in the heel region is from 23 mm to 25 mm.4. The item of footwear of claim 1, wherein the mid-sole in thefrontal/toe region in the part for supporting the ball of the wearer'sfoot is between 12 mm and 16 mm less than the thickness of the mid-solein the heel region.
 5. The item of footwear of claim 1, furthercomprising a recess formed in a medial side surface of the midsole whichreduces in width the midsole at a lowermost surface of the midsole, withthe width of the midsole narrowing downwardly from an uppermost surfaceof the midsole to the lowermost surface of the midsole.
 6. The item offootwear of claim 5, wherein the recess narrows the lowermost surface ofmidsole to a narrowest width that is up to 14 mm less than a narrowestwidth of the uppermost surface of the midsole.
 7. The item of footwearof claim 5, wherein the recess narrows the lowermost surface of themidsole such that the lowermost surface has a narrowest width which isbetween 80% and 90% of a narrowest width of the uppermost surface of themidsole.
 8. The item of footwear as claimed in claim 5, wherein therecess extends along the medial side of the midsole from a point in thefrontal/toe region of the sole aft of a front of the sole rearwardlyacross the whole of the mid-foot region of the sole to a point in theheel region of the sole forward of the rear of the heel.
 9. The item offootwear as claimed in claim 5, wherein the recess is a concave recess.10. The item of footwear of claim 1, wherein the sole further comprisesan outsole, for engaging the ground.
 11. The item of footwear of claim1, wherein the sole further comprises a topsock provided on an/theuppermost surface of the midsole and engageable by a foot of the wearer.12. The item of footwear of claim 1, wherein the sole further comprisesan insole provided on an/the uppermost surface of the midsole andengageable by a foot of the wearer.
 13. The item of footwear of claim 1,wherein mid-sole thickness of 18 mm to 28 mm in the heel region is theminimum thickness of the mid-sole in the heel region.
 14. The item offootwear of claim 1, wherein the mid-sole thickness of 18 mm to 28 mm inthe heel region is the average thickness of the mid-sole in the heelregion.
 15. The item of footwear of claim 1, wherein the frontal/toeregion of the sole is the forwardmost 15-24% of the sole, the mid-footregion is the middle 37-53% of the sole and the heel region is therearmost 29-39% of the sole.
 16. The item of footwear of claim 1,wherein in the heel region, an/the uppermost surface of the midsole isof greater surface area than a/the lowermost surface of the midsole, aperiphery of the midsole being stepped in between 2 mm to 6 mm from theuppermost to lowermost surfaces.
 17. An item of footwear comprising: asecuring means for securing the item of footwear to a foot of a wearer;and a sole having a midsole which has a frontal/toe region, a mid-footregion and a heel region, the mid-foot region of the midsole beingformed from a material different to the material used for the heelregion such that the mid-foot region has a lower compressive resistanceand/or a lower density and/or a higher flexibility than the heel region,wherein the maximum width of the upper surface of the mid-sole in theheel region is in the range of 55 mm to 68 mm.
 18. An item of footwearcomprising: a securing means for securing the item of footwear to a footof a wearer; and a sole having a midsole which has a frontal/toe region,a mid-foot region and a heel region, the mid-foot region of the midsolebeing formed from a material different to the material used for the heelregion such that the mid-foot region has a lower compressive resistanceand/or a lower density and/or a higher flexibility than the heel region,wherein the maximum width of the upper surface of the mid-sole in thefrontal/toe and arch regions is in the range of 80 mm to 92 mm.
 19. Theitem of footwear of claim 18, wherein the securing means is a cover forencapsulating the toes of the wearer.
 20. An item of footwearcomprising: a securing means for securing the item of footwear to a footof a wearer; and a sole having a midsole which has a frontal/toe region,a mid-foot region and a heel region, the mid-foot region of the midsolebeing formed from a material different to the material used for the heelregion such that the mid-foot region has a lower compressive resistanceand/or a lower density and/or a higher flexibility than the heel region,wherein: the sole in the heel region has a thickness in the range of 18mm to 28 mm; and the sole in the frontal/toe region has a thickness in apart of the region for supporting a ball of wearer's foot which isbetween 6 mm and 10 mm less than the thickness of the sole in the heelregion.
 21. An item of footwear comprising: a securing means forsecuring the item of footwear to a foot of a wearer; and a sole having amidsole which has a frontal/toe region, a mid-foot region and a heelregion, the mid-foot region of the midsole being formed from a materialdifferent to the material or materials used for the frontal/toe regionand the heel region such that the mid-foot region has a lowercompressive resistance and/or a lower density and/or a higherflexibility than the frontal/toe region and the heel region, wherein:the sole in the heel region has a thickness in the range of 18 mm to 28mm; and the sole in the frontal/toe region has a thickness in a part ofthe region for supporting a ball of wearer's foot which is between 6 mmand 10 mm less than the thickness of the sole in the heel region. 22.The item of footwear of claim 20, wherein the thickness of the sole inthe heel region is 23 mm.
 23. The item of footwear of claim 20, whereinthe sole in the frontal/toe region in the part for supporting the ballof the wearer's foot is 8mm less than the thickness of the sole in theheel region.
 24. The item of footwear of claim 20, further comprising arecess formed in a medial side surface of the midsole which reduces inwidth the midsole at a lowermost surface of the midsole, with the widthof the midsole narrowing downwardly from an uppermost surface of themidsole to the lowermost surface of the midsole.
 25. The item offootwear of claim 24, wherein the recess narrows the lowermost surfaceof midsole to a narrowest width that is 14 to 20 mm less than anarrowest width of the uppermost surface of the midsole.
 26. The item offootwear of claim 24, wherein the recess narrows the lowermost surfaceof the midsole such that the lowermost surface has a narrowest widthwhich is between 70% and 80% of a narrowest width of the uppermostsurface of the midsole.
 27. The item of footwear as claimed in claim 24,wherein the recess extends along the medial side of the midsole from apoint in the frontal/toe region of the sole aft of a front of the solerearwardly across the whole of the mid-foot region of the sole to apoint in the heel region of the sole forward of the rear of the heel.28. The item of footwear as claimed in claim 24, wherein the recess is aconcave recess.
 29. The item of footwear of claim 20, wherein the solefurther comprises an outsole, for engaging the ground.
 30. The item offootwear of claim 20, wherein the sole further comprises a topsockprovided on an/the uppermost surface of the midsole and engageable by afoot of the wearer.
 31. The item of footwear of claim 20, wherein thesole further comprises an insole provided on an/the uppermost surface ofthe midsole and engageable by a foot of the wearer.
 32. The item offootwear of claim 20, wherein sole thickness of 18 mm to 28 mm in theheel region is the minimum thickness of the sole in the heel region. 33.The item of footwear of claim 20, wherein the sole thickness of 18 mm to28 mm in the heel region is the average thickness of the sole in theheel region.
 34. The item of footwear of claim 20, wherein thefrontal/toe region of the sole is the forwardmost 15-21% of the sole,the mid-foot region is the middle 47-53% of the sole and the heel regionis the rearmost 20-35% of the sole.
 35. The item of footwear of claim20, wherein an/the uppermost surface of the midsole is of greatersurface area than a/the lowermost surface of the midsole, a periphery ofthe midsole being stepped in between 1.5 mm to 2.5 mm from the uppermostto lowermost surfaces.